Flash Flood Warning 2, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1158 AM MDT Wed Jun 7 2023

Aviation Section Updated.

.SYNOPSIS...

Daily showers and thunderstorms move through the area for the next
several days, with the most rainfall forecast to occur tomorrow
and Friday. Considering our wet and saturated soils, flooding
continues to remain a concern.

&&

.UPDATE...

Showers and thunderstorms are already initiating along the
Continental Divide and over the higher terrain Central/Southwest
MT. Today's storms should mostly stay southeast of a KHLN to KHVR
line and over the higher terrain along the Divide. PWATS in the
0.80 to 1 inch mark will increase the potential for heavy
downpours in excess of 1 inch in short periods of time. Otherwise,
gusty winds, hail, and frequent lightning will be the primary
hazards. The best chance for severe wind gusts (> 58 mph)/hail (>
1 inch diameter) will be south of I90 and over the far eastern
portions of North- central MT. Minor changes were made to the
forecast per current trends. - RCG

&&

.AVIATION...
1158 AM MDT Wed Jun 7 2023 (07/18Z TAF Period)

VFR conditions prevail early this afternoon. However, convection is
already beginning to develop over the higher terrain of Central and
Southwest Montana. Thunderstorms are expected (70% confidence) to
impact some Southwest terminals late this afternoon or evening, to
include KBZN/KEKS/KWYS/KHLN. Where thunderstorms do occur near the
airfield, expect temporary MVFR/IFR conditions in heavy rain, along
with variable gusty winds and small hail. Thunderstorm coverage
decreases overnight, but overall mid-level SCT/BKN clouds will
persist, with rainshowers and possible thunderstorms returning mid-
day Thursday.

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.

&&

.HYDROLOGY...

A Flood Watch for Flash Flooding is valid from tomorrow at noon
through late Friday night across much of North Central Montana,
along with portions of the Southwest.

Recent rainfall across the region has resulted in very wet to
saturated soils for much of the area. These saturated soils are
already resulting in some minor flooding. Fergus County Emergency
Management reports flooding of basements and low lying areas.

Additional rain is expected this week, particularly during the
period of the aforementioned Flood Watch, as a low pressure system
moves into the area. This low brings with it widespread moderate to
heavy rain, at times, with many areas included in the watch,
forecast to receive at least a half an inch to an inch, or more,
of precipitation. Some areas along the Rocky Mountain Front and
the adjacent plains are forecast to receive one and a half inches,
or more, of rainfall over a 48 hour period. In addition, embedded
areas of thunderstorms within this system, could drop a quick
half inch to an inch of rainfall over the course of an hour, or
so. Flash flooding could result from the moisture produced by any
of these thunderstorms that move over the already saturated soils.
- Ludwig / Fogleman

&&

.PREV DISCUSSION...
/ISSUED 544 AM MDT Wed Jun 7 2023/

Note: The forecast area of responsibility for the National
Weather Service Great Falls Weather Forecast Office includes North
Central, Central and Southwest Montana.

This forecast period begins with an upper level closed area of
lower heights located over the Canadian Northwest Territories,
while a second upper level closed lower height feature is located
over Southern California. Additionally, an upper level trof
stretches from The Alaskan Aleutian Islands through and along the
Canadian West Coast. These systems progress eastward, and The
Treasure State begins to feel the impacts.

Another round of showers and thunderstorms is forecast for today.
The Excessive Rainfall Outlook includes our entire region in a
Marginal Risk (at least 5% chance) for excessive rainfall.
Additionally, the Severe Weather Outlook includes Southwest Montana
in a Marginal Risk for Severe Thunderstorms, while the remainder of
our area of responsibility falls under the mention of General
Thunderstorm activity. Some of today's thunderstorms could produce
strong erratic gusty winds, brief heavy downpours and hail.

Thursday finds a retrograding ridge building back across Montana,
causing the Canadian Northwest Territories upper level lower heights
to, rapidly, move eastward across Northern Canada, while the
California upper level low moves northeastward along the leading
edge of the ridge. The resultant upper level southeastward flow
advects abundant moisture into our region from The Gulf of Mexico.
Thursday and Friday, much of our region is included in a Slight Risk
(at least 15% chance) area for Excessive Rainfall, while the Severe
Weather Outlook places us in a Marginal Area for Severe
Thunderstorms on Thursday, and a General Thunderstorm Severe Weather
Outlook for Friday. Considering the amount of moisture that our area
has received, recently, our soils are very moist to saturated;
therefore, flooding is a concern that we will be monitoring,
closely. Please, see the HYDROLOGY section, below, for more details.
During the day on Friday, the lower heights originating over
California begin to feel the upstream trof, take a northward turn,
and become absorbed into said trof, ultimately, taking another dive
southeastward along the California Coastline, and a pattern that
favors unstable and active conditions sets up for our region.

Heading into this weekend and through the remainder of this
period, daily showers and thunderstorms are forecast; however,
with time, there is lower confidence of impacts to hydrology.

Throughout this period, daily high temperatures, generally, range
in the 70s.
- Fogleman

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  79  57  77  58 /  20  10  90  90
CTB  80  55  77  57 /  10  10  90  90
HLN  83  60  78  59 /  40  40 100  90
BZN  80  52  76  50 /  40  50  90  80
WYS  76  42  67  39 /  60  60  90  50
DLN  77  52  70  49 /  40  50  90  60
HVR  81  59  82  62 /  30  10  80  80
LWT  75  54  76  55 /  70  50  90  90

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Thursday afternoon through late Friday night
for Bears Paw Mountains and Southern Blaine-Big Belt, Bridger
and Castle Mountains-Cascade County below 5000ft-East Glacier
Park Region-Eastern Glacier, Western Toole, and Central Pondera-
Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-
Fergus County below 4500ft-Gates of the Mountains-Hill County-
Judith Basin County and Judith Gap-Little Belt and Highwood
Mountains-Meagher County Valleys-Northern Blaine County-Northern
High Plains-Snowy and Judith Mountains-Southern High Plains-
Southern Rocky Mountain Front-Western and Central Chouteau
County.

&&

http://www.weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
1259 PM MDT Wed Jun 7 2023

MTZ313-314-318-071930-
Snowy and Judith Mountains MT-Judith Basin County and Judith Gap MT-
Fergus County below 4500ft MT-
1259 PM MDT Wed Jun 7 2023

...Strong thunderstorms will impact portions of south central Fergus
County through 130 PM MDT...

At 1256 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 10 miles southeast of Lewistown to 9 miles
southeast of Moore. These storms were nearly stationary.

HAZARD...Winds in excess of 30 mph and nickel size hail.

SOURCE...Radar indicated.

IMPACT...Localized flooding and Gusty winds could knock down tree
         limbs and blow around unsecured objects.

Locations impacted include...
Lewistown, Moore, Glengarry, Heath and Forestgrove.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with these storms and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 4706 10901 4713 10970 4683 10972 4678 10926
TIME...MOT...LOC 1856Z 246DEG 11KT 4697 10926 4689 10954

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...30 MPH

RCG